How do I update the weather so I have the most recent information?

Using an ADS-B or XM receiver with ForeFlight is the best way to ensure that you have up to date weather while in flight.

If you don't have an ADS-B or XM receiver, then you will want to use the Pack feature just before you disconnect from WiFi to go fly. The Pack feature will analyze your proposed route and look for the most up to date data for various items, including weather products.  You can learn more in the article:

      What is the Pack feature?